Not applicable.
This invention relates to upstream precoders for digital signals. More particularly, the invention relates to a precoder structure to account for nonuniform code set levels compensating for nulls in a system utilized to transmit a digital signal, allowing spectral and constellation shaping.
For V.92, it has been agreed that a xe2x80x9cPCM upstreamxe2x80x9d scheme will be used. This means that the analog modem transmits a signal from a constellation of points that is received at the central office (CO) codec at predetermined levels. This is different than PCM downstream (as in V.90) in part because the channel and noise come before the CO codec. Unlike downstream where any sequence of levels is possible, it is not possible, given power constraints, to make arbitrary decisions about the sequence of levels that will be received by the CO codec in the upstream direction. As a simple example, it is not possible to continuously transmit the same level since the modem channel does not pass a DC component. In addition, it is not possible to do any equalization before the CO codec.
3Com, Conexant Systems, and Motorola ING have proposed using a generalized Tomlinson-Harashima (TH) precoder followed by a linear FIR equalizer in the transmitter. The combination of the precoder and linear equalizer compensate for the channel response so that the levels received by the codec match the levels transmitted.
Unlike V.34, the transmitted upstream symbols in V.92 must be at a symbol rate of 8kHz in order to match the CO codec. This is true regardless of the channel response. The channel, however, does not typically pass either a DC or a Nyquist component. From results in information theory, the spectrum of the transmitted signal should be shaped to match the channel.
Exemplary methods for communication using constellation precoding, Tomlinson precoders and trellis decoder are found in the prior art, including in U.S. Pat. No. 5,388,124 to Laroia U.S. Pat. No. 6,026,120 to Betts, U.S. Pat. No. 5,881,108 to Herzberg and U.S. Pat. No. 5,973,725 to Lee.
The Laroia-Tretter-Farvardin (LTF) precoder structure of the present invention will allow spectral shaping at the transmitter whereas a TH precoder will not. The present invention teaches that an LTF precoder can be applied to V.92 to allow spectral shaping and potential constellation shaping. The output of a TH precoder has an approximately uniform probability density function and samples are approximately independent. Its spectrum is flat regardless of the spectrum of the input sequence. The output of the LTF precoder is the original input plus a small uniformly random component. Therefore, the spectrum is largely preserved. By preserving a greater match to the original spectrum, decreased power is required because the power which is used is concentrated where the channel has the least attenuation and can pass the signal.
The preferred exemplary embodiment teaches a generalized LTF precoder for V.92 that allows non-uniform constellations, spectral shaping, and constellation shaping. The generalized LTF precoder itself is a change from proposals already considered and includes framing, trellis coding, and constellation design.